Mankind's ancestors may have started walking on two legs simply because it allowed them to carry more food away in their hands, boosting their chance of survival, scientists believe.
Anthropologists studying chimpanzees found that the great apes, who usually walk on all fours, walk upright and free their hands for carrying when they need to monopolise hard-to-find resources by swiping more at a single attempt in the face of fierce competition.
The team from the University of Cambridge and Kyoto University in Japan believe the benefit of "first come, first served" and getting a bigger share of scarce food supplies could, over a long period of time, have led some of our earliest "hominin" ancestors to evolve into "bipedal" primates walking on two legs permanently instead of four.
Professor William McGrew, from Cambridge's department of archaeology and anthropology, said: "Bipedality as the key human adaptation may be an evolutionary product of this strategy persisting over time. Ultimately, it set our ancestors on a separate evolutionary path."
Scientists believe that man's ancestors changed how they moved at a time of climate upheaval which reduced the forested areas in which they lived and forced them out into the open more. But a lack of fossils means there is division over what specific factor it was that led to the development of walking on two feet.
The research by the team led by PhD student Susana Carvalho and Professor Tetsuro Matsuzawa, published today in the journal Current Biology, suggests our earliest hominin ancestors may have lived in shifting environmental conditions in which certain resources were not always easy to come by.
Chimpanzees are one of several ape species able to walk on two legs for short periods of time.
The scientists conducted two studies of chimpanzees in Bossou Forest in Guinea, west Africa, finding that when supplies of highly prized coula nuts were scarce, the chimps were more likely to walk on two feet in an attempt to carry off more in a single trip.
They also found that when the chimpanzees went "crop raiding", 35% of their activity involved some sort of bipedal movement, and "once again, this behaviour appeared to be linked to a clear attempt to carry as much as possible in one go".
By studying the behaviour of chimpanzees, they believe that over time, intense bursts of bipedal activity in early hominins may have led to anatomical changes that in turn became the subject of natural selection where competition for food or other resources was strong.
In the paper, the scientist wrote: "Foods may be worth carrying if they are unpredictable in availability. More efficient access to resources may be crucial given this uncertainty (ie 'If I return later, will any resource be left?'). For Bossou's chimpanzees, these factors combine to increase ... the number of items carried at a time, apparently in order to optimise time and energy intake through reduced competition.
"The potential benefit of 'first come, first served' access to resources may lead to using extra parts of the body as 'containers', shifting to bipedal locomotion to free the hands, affording greater carrying capacity."
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